This invention relates to thermoplastic foams and foam articles having one or more of the following properties: small cells, uniform cell sizes, pressure sensitive adhesive compositions, blended immiscible thermoplastic polymer compositions. The invention further relates to a method for making the foams and a method for coextruding the foams with other materials.
In one aspect, the present invention relates to continuous processes for producing foams. The processes can be used to produce foams comprised of amorphous thermoplastic polymers, including pressure sensitive adhesives, and blends of immiscible polymers. Another aspect of the invention is a process to coextrude the foams with other polymeric materials.
In one aspect, the present invention provides a continuous method for producing a foam material comprising:
(1) mixing at least one amorphous thermoplastic polymeric material and at least one physical blowing agent in an apparatus having an exit shaping orifice at a temperature and pressure sufficient to form a melt solution wherein the blowing agent is uniformly distributed throughout the polymeric material;
(2) reducing the temperature of the melt solution at the exit of the apparatus to an exit temperature that is equal to or less than 30xc2x0 C. above the glass transition temperature of the neat polymeric material while maintaining the melt solution at a pressure sufficient to keep the blowing agent in solution; and
(3) passing the solution through the exit shaping orifice and exposing the solution to atmospheric pressure, thereby causing the blowing agent to expand resulting in nucleation and cell formation, which causes the melt solution to foam at or about the time it exits the shaping orifice.
In another aspect, the invention provides foam-containing articles that can be designed to exhibit a wide range of properties for a myriad of applications. The polymeric materials used in making the articles may comprise amorphous thermoplastic polymers including pressure sensitive adhesives, and blends of immiscible thermoplastic polymers. A range of suitable exit temperatures may be determined based on the polymeric material used to make the foam.
In another aspect, the invention further provides a way to control the cell size and cell size distribution of a foam by adjusting, manipulating, or controlling the blowing agent concentration, the exit temperature, and/or the exit pressure of the foamable melt solution.
In another aspect, the invention features articles comprising a foam having cell sizes of 2 to 200 micrometers, preferably 5 to 50 micrometers. The foam may alternatively, or additionally, have a cell size distribution with a polydispersity from 1.0 to 2.0, preferably from 1.0 to 1.5, more preferably from 1.0 to 1.2.
In another aspect, the invention features articles wherein the foam of the invention comprises at least one layer in a multi-layer construction.
The invention further features a coextrusion process whereby a foam is coextruded with at least one other material, which may be a foamed or unfoamed material.
As used in this invention:
xe2x80x9csmall-cell foamxe2x80x9d means a foam having cell sizes of 2 to 200 micrometers (xcexcm), preferably 5 to 50 xcexcm;
xe2x80x9cclosed-cellxe2x80x9d means a foam material that contains substantially no connected cell pathways that extend from one outer surface through the material to another outer surface;
xe2x80x9coperating temperaturexe2x80x9d means the temperature that must be achieved in the extrusion process, prior to the addition of the physical blowing agent, to melt all of the polymeric materials in the melt mix;
xe2x80x9cTgxe2x80x9d means the glass transition temperature, i.e., the temperature at which a polymer changes from a fluid to a solid state;
xe2x80x9cexit temperaturexe2x80x9d and xe2x80x9cexit pressurexe2x80x9d mean the temperature and pressure of the extrudate in the final zone or zones of the extruder and preferably in the die;
xe2x80x9caveragexe2x80x9d means the arithmetic average, i.e., mean;
xe2x80x9cstandard deviationxe2x80x9d means the xe2x80x9ctypicalxe2x80x9d deviation in cell size from the mean cell size; it is calculated using the following formula:   σ  =                              ∑                      i            =            1                    n                ⁢                              (                                          x                i                            -                              x                _                                      )                    2                            n        -        1            
xe2x80x83where "sgr" is the standard deviation, xi is an observed cell size, {overscore (x)} is the arithmetic average cell size, and n is the total number of cell size observations;
xe2x80x9cmelt solutionxe2x80x9d or xe2x80x9cmelt mixturexe2x80x9d or xe2x80x9cmelt mixxe2x80x9d means a melt-blended mixture of polymeric material(s), any desired additives, and blowing agent(s) wherein the mixture is sufficiently fluid to be processed through an extruder;
xe2x80x9cneat polymerxe2x80x9d means a polymeric material having no additives, and at standard temperature and pressure;
xe2x80x9cnucleationxe2x80x9d means a process by which a homogeneous solution of polymeric material and dissolved molecules of a species that is a gas under ambient conditions undergoes formations of clusters of molecules of the species that define xe2x80x9cnucleation sitesxe2x80x9d from which cells will grow; i.e., it is a change from a homogeneous solution to a multi-phase mixture in which, throughout the polymeric material, sites of aggregation of at least several molecules of physical blowing agent are formed (if immiscible polymeric materials are used, the physical blowing agent will typically form single-phase solutions with one or more of the polymer materials, but the polymers will typically not combine to form a single phase);
xe2x80x9csupercritical fluidxe2x80x9d means a substance, which is typically a gas at ambient temperature and pressure, compressed to a state where it has the density and solvation characteristics of a liquid, but the viscosity, permeability, and diffusivity of a gas; a supercritical fluid is a single phase material that exists above a critical point, which point is determined by a critical temperature, Tc, and critical pressure, Pc, which Tc and Pc depend on the particular gas (for example, the Tc and Pc for carbon dioxide are approximately 31xc2x0 C. and 7.4 MPa (1078 psia), respectively);
xe2x80x9cfoam densityxe2x80x9d means the weight of a given volume of foam;
xe2x80x9cinversion temperaturexe2x80x9d means the temperature at which a minimum foam density is obtained for a given polymeric foam; at temperatures above and below the inversion temperature, a higher foam density will typically be obtained;
xe2x80x9cdensity reductionxe2x80x9d refers to a way of measuring the void volume of a foam based on the following formula:       ρ    R    =            ⌊              1        -                              ρ            f                                ρ            o                              ⌋        xc3x97    100    ⁢    %  
xe2x80x83where xcfx81R is the density reduction, xcfx81f is the foam density, and xcfx81o is the density of the original material;
xe2x80x9cpolydispersityxe2x80x9d means the weight average cell diameter divided by the number average cell diameter for a particular foam sample; it is a means of measuring the uniformity of cell sizes in the sample;
xe2x80x9cuniformxe2x80x9d means that the cell size distribution has a polydispersity of 1.0 to 2.0;
xe2x80x9csphericalxe2x80x9d means generally rounded; it may include spherical, oval, or circular structure;
xe2x80x9cfibrillosexe2x80x9d means having elongated filament-like or thread-like structures;
xe2x80x9cschistosexe2x80x9d means having parallel plate-like ribbons;
xe2x80x9cpolymer matrixxe2x80x9d means the polymeric, or xe2x80x9cnon-cell,xe2x80x9d areas of a foam;
xe2x80x9cblend matrixxe2x80x9d means the polymeric material having the highest volume fraction in a melt mixture comprising at least two immiscible materials;
xe2x80x9cimmisciblexe2x80x9d refers to thermoplastic polymers that will not mix or remain mixed with each other, although at certain conditions, such as high temperatures, they might mix, but any such mixture will typically be thermodynamically unstable and will typically separate into distinct- phases at lower temperatures;
xe2x80x9cmisciblexe2x80x9d refers to two or more thermoplastic materials that will form a homogeneous mixture, that is, dissolve in each other;
xe2x80x9canisotropicxe2x80x9d means having different properties or degrees of properties in different directions parallel to a major surface; and
xe2x80x9cstraight line tearxe2x80x9d means a tear not deviating more than 20xc2x0, preferably not more than 10xc2x0, from the direction in which the tear is initiated.
An advantage of at least one embodiment of the present invention is the ability to alter, adjust, or control the foam density, average cell size, and cell size distribution of foams. This allows the properties of the produced foams to be optimized based on their intended use.
An advantage of at least one embodiment of the method of the present invention is that no special nucleation apparatus is required to nucleate the foam.
An advantage of at least one embodiment of a foam of the present invention is that uniform cell sizes can provide uniform characteristics and properties throughout the foam.
An advantage of at least one embodiment of a foam of the present invention is that small cells, as opposed to larger cells, will not as easily propagate defects or cracks in the foam structure. Another advantage of small cell sizes is that thinner foam substrates can be produced.
An advantage of at least one embodiment of a foam of the present invention is that a foam comprising a blend of thermoplastic materials can allow the foam to have beneficial properties of each material. For example, a foam combining a stiff and strong material with a flexible and weak material can have strength and flexibility. In addition, the foam may have anisotropic properties, which can be used advantageously. For example, a foam that has high tensile strength in one direction and flexibility in a perpendicular direction may be used for high strength substrates or backings.
Another advantage of at least one embodiment of a foam of the present invention is that the foams may have low and controlled dielectric constant and high dielectric strength.
An advantage of at least one embodiment of a foam of the present invention is the ability to be torn by hand in one or both of directions parallel and perpendicular to the machine direction wherein the tears are straight-line tears.
An advantage of at least one embodiment of the present invention comprising pressure sensitive adhesive materials is that the foam can possess pressure sensitive adhesive properties without requiring the application of a pressure sensitive adhesive layer to the foam surface.
An advantage of at least one embodiment of the present invention is the ability to produce coextruded foams from materials having disparate temperatures, while maintaining the structure of the foam. The coextruded foams can provide articles having a variety of desired properties.
An advantage of at least one embodiment of a foam of the present invention is that it can provide the insulating properties.
An advantage of at least one embodiment of a foam of the present invention is that it can provide the conformability and strength of a cloth without using an expensive woven material.
Other features and advantages of the invention will be apparent from the following figures, detailed description, and claims.